There are several known processes for the production of water lacquers from alkyd resins. These known processes can be divided into two main types. One type of alkyd resin can be made up into aqueous dispersions using emulsifiers or hydrophilic resins (emulsifier resins). Dispersions such as these generally contain no auxiliary solvents. However, they have the disadvantage of inadequate resistance to water; in other words, the lacquer film shows a tendency towards redispersion even after drying, for example by stoving. In most cases, it is also not possible to produce high-gloss lacquer films with such resins.
The second process type for producing alkyd resin water lacquers comprises dissolving alkyd resins after neutralization. In most cases, this involves the use of considerable quantities of organic solvents, which offsets the main advantage of water lacquers, namely, their lack of harmful effects on the environment.
Another method for producing alkyd resin water lacquers comprises the subject matter of German patent application No. 28 42 919. In this known process, alkyd resins having acid numbers of from 30 to 180 mg of KOH/g are initially prepared and then esterified by the action of glycidol up to an acid number of from 5 to 35, preferably from 15 to 30. The resins thus produced, which contain 2,3-dihydroxypropyl groups, are then neutralized with substantially nonvolatile amino alcohols and made up into water lacquers with the assistance of emulsifiers which lose their hydrophilicity on stoving. A special variant of this process is described in German patent application No. 28 42 919. According to that variant, resins containing 2,3-dihydroxypropyl groups can also be produced by the action of glycerol on alkyd resins having a high acid number.
Accordingly, resins containing 2,3-dihydroxypropyl groups have the advantage that they can be made up into water lacquers without any need to use volatile constituents, such as solvents or even volatile amines. However, emulsifiers still have to be used. Although emulsifiers which lose their hydrophilicity on stoving, i.e. heat-reactive emulsifiers which are split into fragments at elevated temperatures, or emulsifiers which are incorporated in the resin by stoving through hydroxyl groups, have been proposed for this purpose, both these types of emulsifiers can produce faults in the lacquer film such as yellowing as a result of chemical reaction during stoving; the exuding of fragments, i.e. fragments collect at the interface with the lacquered substrate and, hence, interfere with adhesion; or because overly narrow crosslinking occurs, resulting in embrittlement under stoving conditions due to the presence of numerous hydroxyl groups. Thus, glycerol-modified resins in particular are attended by the disadvantage of minimal processing tolerance, which means that coatings combining adequate hardness with adequate flexibility can only be obtained under very specially selected stoving conditions. However, where lacquering is carried out industrially on the assembly-line principle, stoving conditions often cannot be kept constant enough to produce uniformly acceptable results.
Additionally, the starting material, glycidol (2,3-epoxypropanol), is difficult to handle. Hence, there is a need for new alkyd resins which can be produced by a process that does not involve the use of glycidol and which results in alkyd resins that can be made up into water lacquers without the assistance of organic solvents or emulsifiers.